The control of the gas generated in landfills is a serious problem. Through the naturally occurring decomposition processes that occur within the waste contained in the landfill, a number of gaseous products are generated. These gaseous products are primarily methane and carbon dioxide in roughly equal quantities, and to a lesser degree nitrogen (&lt;5% ). These gaseous products, if allowed to migrate uncontrolled from a landfill, may result in dangerous conditions within buildings that the gas may enter. Uncontrolled release of the gas to the atmosphere may cause air pollution.
Landfill gas composition is controlled primarily by products of microbial reactions in the landfill. In general, a landfill will go through several different stages with different bacterial types predominating in each stage.
Solid waste initially decomposes aerobically, and the primary gas product is carbon dioxide. As the oxygen is depleted, aerobic microorganisms begin to dominate. These bacteria continue to produce carbon dioxide, but additionally, produce methane. Additional compounds are produced, and additional chemicals released into the landfill by volatilization.
The need for gas movement control is primarily to prevent the gas from damaging plants and property. Methane generated in landfills has the potential for damaging vegetation, as it displaces oxygen from the root zone.
The mechanics of gas movement through refuse and soil are extremely complicated. The gas will tend to migrate from the landfill on a path that offers the least resistance. Gas will migrate further through a sand and gravel soil than through a silt or clay soil. The rate of migration is strongly influenced by weather conditions. As the barometric pressure falls, gas will tend to be forced out of the landfill into the surrounding formations. As the pressure rises, gas may be retained within the landfill for a short time period as new pressure balances are established.
Controlling gas movement at a landfill begins with a study of the local soils, geology and nearby area. Typically, landfills are covered by a combination capping system. This system can include a series of bentonite clays and a polymeric (typically polyvinyl chloride (PVC)) liner. This liner system has the dual purpose of excluding moisture infiltration into the landfill, allows for proper management of surface water by directing infiltrated water to sedimentation/stormwater retention basins and to contain landfill gas and odors. The latter effect of polymeric liners requires that gas which collects under the liner be directed to vents so as to minimize lateral migration of the gas, into potentially occupied structures.
Passive vents and active gas pumping systems are used to control landfill gas migration. Passive systems rely on natural pressure and convection mechanisms to vent the landfill gas to the atmosphere. Shallow gas venting trenches, or gas venting pipes, installed within the landfill and vented to the atmosphere, have been used to allow gas from interior regions of the landfill to escape. These natural vents may be equipped with flares to burn off the gas in order to prevent odor problems.
Passive vents however suffer from a number of defects. In general, they are not very effective in removing the landfill gas from under the cover, resulting in vegetative stress. This failure is generally attributed to the fact that there is insufficient pressure on the gas within the landfill to push it to the venting device. An additional problem with the passive design lies in that alternating periods of high and low barometric pressure result in atmospheric air entering the landfill when barometric pressure rises.
Active gas collection systems remove the landfill gas under a vacuum from the landfill or surrounding soil formation, with the gas being pumped out of the ground. A pipe network is built to interconnect the wells and blower equipment. Recovery wells are constructed near the perimeter of the landfill. Depending on site conditions, the wells may be placed in the waste or in the soil formation immediately adjacent to the landfill The location will depend on site access, the type of soil formation around the site, and the type of waste in the landfill.
Borehole diameters are generally two to three feet. Larger diameter holes provide more surface area at the refuse-gravel interface and require less suction for gas removal. This configuration is above ground, therefore providing easy access to piping. An alternative design is for the interconnection between the well and header pipe to be entirely below ground, and is best suited for installation at landfills where all equipment must be out of sight.
Well depths ranging from 50-90% of the refuse thickness are common, except where groundwater conditions are encountered, and then the well is terminated at the water table. The well casing is usually some type of slotted plastic pipe. It is important that the wells be individually valved so the vacuum applied to each well can be regulated. Gas probes are used to monitor the performance of the control wells.
Wells are connected to a collection system that carries the gas to a collection point. Collected landfill gas can be directly vented to the atmosphere, burned, or directed to an energy recovery system.